This invention relates to cast metal-matrix composite materials, and, more particularly, to a process and apparatus for solidifying such a composite material.
Reinforced metal matrix composite materials have gained increasing acceptance as structural materials. Metal matrix composites typically are composed of reinforcing particles such as fibers, grit, powder or the like that are embedded within a metallic matrix. The reinforcement imparts strength, stiffness, wear resistance, and other desirable properties to the composite, while the matrix protects the particles and transfers load within the composite piece. The two components, matrix and reinforcement, thus cooperate to achieve results superior to those that either component could provide on its own.
Twenty years ago, reinforced composite materials were little more than laboratory curiosities because of very high production costs and their lack of acceptance by product designers. More recently, great advances in the production of nonmetallic composite materials, such as graphite-epoxy composite materials, have been made, with a significant reduction in their cost. The cost of metal-matrix composite materials has remained relatively high. In the last several years, the discovery of a processing technology that permits the reproducible production of large quantities of cast reinforced composite materials with metal matrices has significantly reduced the cost of these materials. See, for example, U.S. Pat. No. 4,759,995 and U.S. Pat. No. 4,786,467, whose disclosures are incorporated by reference.
Since the discovery of the methods of the '995 and '467 patents, many applications for cast composite materials have been developed, and their volume of use has increased significantly so that they have become a major new type of structural material. These cast metal matrix composite materials offer the property improvements of composite materials at a cost only slightly higher than that of conventional monolithic materials. The cast metal-matrix composite materials may be used at elevated temperatures or under other conditions that preclude the use of organic-matrix composite materials.
Although the processes of the '995 and '467 patents have provided a major advance in the field enabling the production of cast metal-matrix composite materials on an industrial scale, the composite structures produced by these techniques are not always optimal. For example, it has been observed that in some cases there are irregularities in the microstructures of the composite materials prepared by these approaches. These irregularities are manifested as inhomogeneous regions within the composite material wherein the reinforcement particles are not evenly distributed. Additionally, between the particles the matrix sometimes exhibits a segregated eutectic structure with a reduced melting point. These microstructural irregularities result in degraded physical properties as compared with those expected for a more homogeneous composite material.
Accordingly, there exists a need for an improved cast metal-matrix composite fabrication procedure that produces a uniform microstructure and corresponding improved properties. The present invention fulfills this need, and further provides related advantages.